Selective radio receiving system



Oct. 5, 1937.

L. E. THOMPSON SELECTIVE RADIO RECEIVING SYSTEM Filed April 30, 1936 WQ QKSQ Gttorneg Patented Oct. 5, 1937 eat-stir creme SELECTIVE more RECEIVING SYSTEM Leland E. Thompson, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 30,

4 Claims.

The present invention relates to a selective radio receiving. system of the type disclosed in my copending application, Serial No. 77,253, filed April 30, 1936, for Selective radio receiving systems, and has for its object toprovide means forreducing the effects of interference in a radio receiving system provided with a single diode rectifier type of limiter in connection with broadly tuned signal input circuits.

In limiting radio interference from static and undesired signal waves in connection with a limiter circuit, selectivity must be obtained, otherwise two signals on differing adjacent channels will be received at the same time. The present invention relates specifically to a receiving system employing a limiter wherein the signals are limited by an oscillator at the beat frequency between the oscillator and the incoming signal on the principle that oscillator voltage is equal to or less than the incoming signal voltage, whereby the output beat frequency between the two input voltages may have an amplitude dependent only upon the amplitude of the smaller of the input voltages, which is the oscillator voltage.

In accordance with the invention, a signal limiter circuit is preceded by broadly tunable signal receiving circuits for supplying signals theretoand is followed by a second rectifier of the diode type which is interposed between the limiter circuit and selectively tuned circuits for imparting selectivity to the receiving system.

The invention will be better understood from the following description, when considered in 35 connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, Fig. 1, is a schematic circuit diagram of a receiving system circuit embodying 40 the invention, and

Fig. 2 is a curve showing the relation between the frequency ranges and amplitudes of voltages applied to the limiter circuit forming part of Fig. 1 and the over-all selectivity of the receiving system preceding the limiter circuit.

circuit, adjacent to the cathode, is a coupling 1936, Serial Nb. 77,254

coil It for an oscillator l4 having a variable tap connection l5 for varying the oscillator voltage applied to the series rectifier circuit.

The adjustment of the contact i5 is such that the oscillator voltage may be maintained'at a predetermined value with respect to the incoming signal voltage, and the frequency of the oscillator is such that it lies outside of the frequency response range of the intermediate frequency amplifier and the tuned circuits 5 and 6. This response characteristic is indicated in Fig. 2 by the curve l6 and, in relation thereto, it will be seen that the oscillator voltage which is represented in amplitude and frequency by the line C lies outside of the response range of the receiving circuits. An interference voltage is indicated by the line B in frequency and amplitude, and a received signal wave in amplitude and frequency is indicated by the line A.

In the presence of an interference wave B, the signal corresponding to BA, the beat frequency between B and A, and the signal corresponding to BC, the beat frequency between B and C, will appear in the output circuit of the limiter across the. impedances H and i2, and in the present example the impedance l l is tuned to the beat frequency BC while the impedance i2 is responsive to the beat frequency BA. The two impedances. in series are coupled to a coupling capacitor H and a couplingresistor is with a sec- 1 0nd diode rectifier l9 in circuit with'which is a sharply tuned output circuit 28 coupled through a transformer 2i with a second sharply tuned output circuit 22.

The circuits 2i] and, 22 are tuned to an intermediate frequency resulting from the beat between the desired signal and the oscillator frequency C, and the rectifier [9 serves to remove the interference component B for the reason that the incoming signal, as explained in the copending application referred to, and the oscillator beat frequency signal tend to modulate the 7 beat frequency between the incoming signal and the interference signal, and the output strength is dependent upon the strength of the oscillations derived from the oscillator M and controllable by the means l5.

The present circuit, therefore, includes a beat heterodyne limiter including a diode rectifier which may be subject to interference from strong local signal cause. Broad circuits are used in the receiving system which, otherwise, would block the operation of the limiter, not by saturation but by modulating the desired signal. Since the desired signal is then modulated, it cannot be received and, accordingly, a second demodulator I9 of the'diode rectifier type is directly connected with the limiter circuit, as shown, for the purpose of maintaining the signal output in the presence of interference. The second rectifier or demodulator is then provided. with sharply tuned output circuits which are responsive to the new intermediate frequency resulting from the beat between the intermediate frequency. in thecircuits 5 and 6 and the local oscillator frequency derived from the source l4, which is the beat frequency AC. In this system, the selective circuits 20 and 22 are depended upon to remove the. audio frequency beat signals and other interference which results from the operation of the limiter circuit in conjunction with the rectifier l9. With this arrangement, the adjacent channel selectivity is maintained although receiver circuits ahead of the limiter are broadly tuned to have an over-all response of the nature ofthe curve i6.

Both rectifier devices are preferably diode's for 7 the reason that it is desirable that both should produce straight line rectification. .In the output of the detector 8, there will be two substantially cies which may be present in the output of the.

detector 8 may beneglected, as they are at best only second order magnitude.

It is also desirable that the impedances H and I2 in the output circuit of the rectifier 8 maintain sufiiciently high impedance to both the signals AB and BC, that both voltages therefrom are impressed upon the second rectifier ii).

If the band pass width 'of'the curve iii is in the order of kilocycles, for example,*it is possible that the impedance l2 may be utilized to ofier the necessary impedance to, both AB and BC. I

I claim asmy invention:

1. In a radio receiving system having broadly tunable receiving circuits, the combination a of diode rectifier means connected therewith, means for introducing heterodyne limiter oscillations into said circuit, diode output impedance means broadly tunable to the beat frequency of an interference wave and said oscillations, and an in- -terference wave and a receivedsignal wave, de-

modulator means includingra second diode rectifier coupled to said impedance means, and an output circuit for said second diode rectifier including an output coupling means sharply responsive tov the beat frequency of the received signal wave and said oscillations. Y

"2. In a superheterodyne radio receiver system,

the combination with an intermediate frequency 'amp'lifiero-utput circuit, of means forbroadly tuning said circuit, a diode rectifier-device connested therewith, means for applying oscillations in circuit with said rectifier'to provide a heterodyne signal limiter action with said rectifier, an output circuit for said rectifier including coupling means broadiy responsive to. a beat fre qulency 'resulting from a desired signal, and an interference signal and oscillations from said oscillator, a diode rectifier connected with said output impedance means, saidrectifier' having an output circuit sharply responsive to the beat frequency resulting from'a receivedcarrier wave and said oscillations. V 1' 3. A radio receiving'system, including in combination,"broadly tunable signal receiving circuits terminating in a dio de rectifier circuit adapted for 1 heterodyne' signal limiter action and includinga diode rectifier device, oscillator coupling means in said circuit adjustable to apply oscillations of predetermined amplitude thereto, impedance outcoupling means in said circuit responsive broadly to the beat frequency of an interference Wave and said oscillations, andan interference wave and a received signal wave, a second diode rectifier coupled to said last named coupling means, and an output circuit therefor including tuned transformer coupling meanssharply re- :sponsive to the beat frequency of the receive signal wave and said oscillations. 4. In a radio receiving system having broadly tunable receiving circuits, the combination. of rectifier means connected therewith, means for introducing heterodyne limiter oscillations into said circuit; rectifier output impedance means broadly tunableto the beat frequency of an interference Wave and said oscillations," and an interference wave and a received signal wave, demodulator means including a' second rectifier coupled to saidimpedance means, and an; output circuit for said second rectifier including an output coupling means sharply res'po-nsiveto the beat frequency of the received signal wave and said I oscillations.

V 1 LELAND'E. THOMPSON. 

